Thanks - I'll consider myself warned. I've been mindful of that possibility and have read a few cautionary tales. I'm intending to replace the ceiling with fire bricks before the next bake, and will cycle out the others for fire brick as I can.

As the the specifics - I'm afraid I didn't take any notes or many measurements.... my recollections are:

Bake time... 2:30 to about 3 mins seems about right, I think! Didn't think to stopwatch it, but will attempt to remember next time.

Temperatures... varied a lot, but were about 400-500 on the deck, and 800-900 on the dome.

Undersides... fairly pale. Just as pale as my home oven pizza in fact. Cooked well, but barely browning by the time they were ready on top. No pics, they looked fairly unexciting.

Heat-up time... I think it was about 3-4 hours but I didn't look at the clock once all day. Next firing I'll look to preheat a lot longer to get better heat saturation, especially more into the floor. I think the embers were only just starting to warm up the floor when my patience ran out and I started baking.

Dough... was a 36-ish hour sourdough. I didn't time the dough particularly well as it was guesswork as to when the oven might be ready to cook with.

Clay brick are fired to far higher temps than you are able to reach. They will eventually spall and deteriorate, but not any year soon. Solid soft brick are best, do not use brick with holes in them and you are fine. Concrete brick on the other hand will explosively decompose on being fired.

@Tscarborough - thanks for the info, that's reassuring. I've been reading up on the various types of brick. I've just picked up a ceiling's worth of fire brick, exactly the same spec as the floor. The next bake will use them.

Out of interest, why are the bricks with holes bad? The ceiling that I used for this bake were bricks with bout 8 holes in each brick - presumably to make them able to be reinforced.

Ok interesting. I'll relegate those holey bricks to the outer layers and remove them entirely once I have enough fire brick. I've been looking around at insulation possibilities and may also clad the entire thing in an aluminium shell with a vent/flue. This is a very slippery slope I'm on.